1. Field of the Invention
The invention relates to a ring of an endless metallic belt which is formed by annularly arranging multiple plate-shaped elements that face each other, and passing the ring that is a metallic band through the elements so as to annularly join the elements. More particularly, the invention relates to a rolling apparatus and a rolling method which rolls this ring.
2. Description of the Related Art
In a vehicle, an automatic transmission is installed, which adjusts a speed ratio according to a running state of the vehicle. Examples of such an automatic transmission includes a continuously variable transmission (hereinafter, referred to as “CVT”) which continuously adjusts the speed ratio.
The CVT can efficiently transmit an engine output, and contributes to improving fuel efficiency and running performance. In one of CVTs that are put into practical use, a metallic belt and a pair of pulley are used, and an effective radius of each pulley is changed by a hydraulic pressure such that the speed ratio is continuously changed. In this CVT, an endless metallic belt is wounded on an input side pulley fitted to an input shaft and an output side pulley fitted to an output shaft. Each of the input side pulley and the output side pulley includes a pair of sheaves in which a groove width can be continuously changed. By changing the groove width of each of the input side pulley and the output side pulley, a belt-winding radius with respect to each of the input side pulley and the output side pulley is changed. Thus, a ratio between the rotational speed of the input shaft and the rotational speed of the output shaft, that is, the speed ratio can be continuously changed.
When forming the endless metallic belt, plural types of elements having different thicknesses are prepared, and the plural types of elements at a predetermined ratio of quantity are randomly combined. The endless metallic belt is formed by passing the metallic band through the combined elements. The elements and the ring as the metallic band of the endless metallic belt need to be produced with high dimensional accuracy.
When producing the ring used for the endless metallic belt, both ends of a thin plate of super-strong steel such as maraging steel are bonded by welding such that a cylindrical-shaped drum is formed. Then, the drum is cut at a predetermined width, whereby a metallic ring made of the thin plate is obtained. Then, after the ring is subjected to barrel polishing in order to remove burrs and edges which are generated due to cutting, the ring is rolled so that the ring has a desired thickness.
Toyota Technical Report No. 12000 (published on Feb. 28, 2001) discloses a rolling apparatus which rolls a ring using a pair of rollers. The rolling apparatus performs rolling while changing an interval between the rollers by controlling the positions of the rollers, and calculates the thickness of the ring before rolling based on a speed at which the ring is deformed (stretched) in a peripheral direction at the time of rough rolling, and controls the interval between the rollers or a rolling load according to the calculated thickness of the ring before rolling at the time of finishing rolling.
In this rolling apparatus, the condition for finishing rolling is corrected according to the thickness of the ring which is estimated based on the speed at which the peripheral length of the work piece is stretched at the time of rough rolling, since it takes extremely much time to directly measure the thickness of the ring in microns. When performing rough rolling in which a rolling reduction rate is high, the speed at which the ring is deformed is high, and the speed at which the thickness of the ring is reduced is high. The speed at which the thickness of the ring is reduced is proportional to deformation (stretch) of the ring in the peripheral direction. When the thickness of the ring is slightly changed, the speed at which the ring is deformed in the peripheral direction is greatly changed. Therefore, the speed at which the ring is deformed in the peripheral direction is calculated using a controller based on a detection signal from a tension position detecting sensor, and the condition for finishing rolling is corrected according to the thickness of the ring before rolling which is estimated based on the calculated speed at which the ring is deformed in the peripheral direction.
In the aforementioned rolling apparatus for a ring, only the condition for finishing rolling is corrected based on the speed at which the ring is deformed in the peripheral direction at the time of rough rolling. Since 70% to 80% of the shape of the ring is decided at the time of rough rolling, desired dimensional accuracy may not be obtained only by correcting the condition for finishing rolling. For example, the temperature of the ring before rough rolling may be deviated from a reference temperature (i.e., a reference temperature which is used when the condition for rough rolling and the condition for finishing rolling are decided) due to an increase in the temperature around the rolling apparatus, a high temperature of the ring which is wound on the rolling apparatus, or the like. In such a case, since the condition for rough rolling is greatly changed due to a change in the temperature of the ring, desired dimensional accuracy cannot be obtained by using the aforementioned rolling apparatus, and correcting the condition for finishing rolling according to the thickness of the ring before rolling which is estimated based on the speed at which the ring is deformed in the peripheral direction at the time of rough rolling.